# Synechococcus Under Stress: Contrasting Physiological and Transcriptional Responses to Salinity and Temperature in Marine Versus Euryhaline Strains

## TL;DR

Marine and euryhaline Synechococcus strains respond differently to salinity and temperature stress, with distinct physiological and gene expression strategies.

## Contribution

This study reveals contrasting acclimation strategies in Synechococcus strains under combined salinity and temperature stress.

## Key findings

- The marine strain RS9907 maintained photosynthetic efficiency better than the euryhaline strain WH5701 under cold and salt stress.
- WH5701 showed a stronger transcriptional response to salinity stress, with 71% of genes differentially expressed compared to 6% in RS9907.
- Both strains shared core responses, including upregulation of carbon fixation and osmolyte synthesis genes under stress conditions.

## Abstract

Temperature and salinity are key environmental drivers that constrain growth and distribution of marine cyanobacteria, yet their combined physiological effects remain unexplored. We analysed the physiological and transcriptional responses of two Synechococcus strains, the marine RS9907 and euryhaline WH5701, across a salinity gradient (18–50 PSU) under optimal (28°C–30°C) and low temperature conditions (15°C–20°C). Growth and photosynthetic efficiency (F
V/F
M) declined under salinity stress (18 PSU for RS9907 and 50 PSU for WH5701), relative to typical marine conditions (36 PSU). RS9907 maintained the photosynthetic electron transport rate under salt stress and the F
V/F
M under cold conditions more effectively than WH5701. Salinity induced a stronger regulatory response in WH5701 (71% genes differentially expressed, compared to only 6% in RS9907). Both strains shared a core response, upregulating carbon fixation genes under cold stress, and glycogen degradation and osmolyte synthesis genes at high salinity (42–50 PSU). Conversely, some photosynthetic genes (psbCD, psaC) showed increased expression at low salinity, but temperature‐dependent regulatory differences were observed. WH5701 uniquely upregulated genes related to membrane transporters, fatty acid desaturases and the pentose phosphate pathway within salinity, potentially contributing to their broader tolerance to salt fluctuations. Collectively, our results reveal contrasting strategies of thermohaline acclimation in Synechococcus strains adapted to different salinities.

Two model strains of Synechococcus sp.—the marine RS9907 and the euryhaline WH5701—exhibit distinct physiological responses to salinity after long acclimations at optimal and cold growth temperatures. Transcriptomic and photochemical analyses reveal drastic differences in their regulatory strategies and photosynthetic activity across a salinity gradient, yet highlight shared expression patterns in key functional genes to cope with combined thermal and salinity stress.